1. Field of the Invention
The present invention relates to a creasing device and to an image forming system that includes the creasing device, which, prior to folding a sheet-shaped member (hereinafter referred to as “sheet”) transferred from a preceding device, previously forms a crease in the sheet, and an image forming apparatus.
2. Description of the Related Art
What is called saddle-stitched or center-folded booklet production has been conventionally performed in which a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, is saddle stitched and the thus-saddle-stitched sheet batch is folded in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause an outside sheet of the sheet batch to be stretched at a fold line by a greater amount than an inside sheet. An image portion at the fold line on the outside sheet can suffer damage such as come off of toner caused by being stretched in some cases. A similar phenomenon can occur when other fold, such as z-fold or tri-fold, is performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
Creasing devices, so called creaser, that, prior to a folding process where a sheet batch is folded in half or the like, previously form a crease (score) in sheets to make sheets, including also an outside sheet, easy to be folded, thereby preventing come off of toner have already been known. Such creasing devices typically form a crease in a sheet in a direction perpendicular to a direction, in which the sheet is conveyed, by moving a roller on the sheet, burning the sheet with a laser beam, pressing a creasing blade against the sheet, or a like method.
A known example of such a creasing device is disclosed in Japanese Patent Application Laid-open No. 2009-166928. Disclosed in Japanese Patent Application Laid-open No. 2009-166928 is a technique of moving a creasing member by using a plurality of individually-advancing-and-retracting mechanisms, which move the creasing member at different timings, in order to enable formation of a crease while reducing movement of pressing by the creasing member.
However, forming a crease in a sheet with a roller involves moving the roller across the length of the sheet in a direction, along which a fold line is to lie, and therefore is time consuming. To resolve this, it is conceivable to rotate a sheet conveying direction by 90 degrees and produce a crease parallel to the sheet conveying direction; thereby, time to form a crease becomes unnecessary because the crease can be formed while the sheet is conveyed; however, this scheme involves a change in footprint and therefore is disadvantageous for space-saving design. Creasing by using a laser beam is environmentally less favorable because smoke and odor are given off during creasing.
Creasing a sheet by pressing a creasing blade against the sheet can be performed in a relatively short period of time and allows easy production of a crease perpendicular to a sheet conveying direction; however, pressing a longitudinal face of the creasing blade against the sheet entirely at once can increase a load. To reduce the load, a scheme of bringing the creasing blade face into partial contact with a sheet a plurality of times can be used. However, this scheme is disadvantageous in that unevenness can develop between a portion that contacts the blade multiple times and a portion that contacts the blade only once and also in that producing a crease by making contact multiple times can decrease productivity.
To overcome the inconveniences described above, it is possible to reduce a load placed on a creasing moving unit and cause every part of the creasing blade to contact the sheet only once by bringing the creasing blade gradually into contact with a sheet from an edge of the sheet; however, this causes a pressure applied onto a center portion of the sheet to be weakened, making it difficult to form an even crease. An even crease can be formed by gradually bringing an arcuate creasing blade into contact with a sheet from an edge of the sheet. However, this requires that a channel or a projection having a complex shape be defined in or formed on a curved surface, and thus requires significant time and cost to manufacture the arcuate creasing blade and the arcuate creasing channel.
Therefore, there is a need to enable efficient and low cost manufacture of an arcuate blade that enables formation of an even crease in a sheet.